<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    function foo(){
        console.log(this);
        //this window
        //window.alert() == this.alert()
        this.alert('aa')
    }

    var oo = {
        alert:function(){
            console.log("bb")
        }
    }
//    foo();//相同的 foo.call(window)
//    oo.alert()
    foo.call(oo)
</script>
</body>
</html>